The invention relates to an apparatus for inspecting appearance of a semiconductor device to review defects thereon.
In a manufacturing process of a semiconductor device, in order to ensure a high yield of products, it is necessary to detect and repair various defects occurring in the process at an early stage. This is normally carried out by the following steps.
A semiconductor wafer of interest to be inspected is inspected by a wafer-appearance inspection system, a wafer foreign matter detection system, or the like, to detect the position of a defect occurring on the wafer, or foreign matter attached thereto.
Then, the defect detected is observed (hereinafter referred to as “review”), and classified according to sources. In this review, a device dedicated for defect review, including a microscope, is normally used to observe or review a defective part at high magnification. In some cases, other devices having a reviewing function, for example, an appearance inspection system may be used.
When the number of defects detected by the detection system is very large, the defect review as described above requires much time and effort. Recently, a review apparatus has been increasingly developed which has an automatic defect review (ADR) function of automatically taking images of defects and collecting them. JP-A No. 30652/2000 discloses a review apparatus having the automatic defect review function using a scanning electron microscope in an image pickup system.
FIG. 2 shows a flowchart of an ADR process, which is disclosed in the JP-A No. 30652/2000.
The review apparatus selects one of defects designated, and takes an image of a corresponding part of a chip in a wafer that is adjacent to one chip where the defect part exists, at a first magnification set. This corresponding part has the same pattern as that of the defect part. This image is referred to as a (low-magnification) reference image. The reference image taken is stored in a storage medium (for example, a magnetic disc) in the review apparatus.
Then, a stage is moved such that the position of the defect is located approximately at the center of a visual field of an optical system. Thereafter, an image of the defect part is taken at the first magnification set in the same manner as in taking the reference image. This image is hereinafter referred to as a (low-magnification) defect image.
From the thus-obtained defect image and reference image, the defect part is identified, and then imaged at higher magnification than the first magnification set. This image is hereinafter referred to as a high-magnification defect image. This high-magnification defect image is intended to be used in an automatic defect classification (ADC) at a later stage. The high-magnification defect image is not necessarily essential to identify the defect part, but is needed to the review apparatus for the purpose of reviewing details of the defect.
Identifying the defect part is carried out by a comparison inspection which involves comparing the defect image to the reference image, and extracting a different part between these images as the defect. The comparison inspection can be categorized into two following methods. First, die-to-die comparison involves comparing an image of appearance of a part of one chip to that of another part of a different chip designed to have the same shape on the same wafer. The image of another part serves as a reference image. As the other comparison method, cell-to-cell comparison involves, when the same wiring pattern is periodically designed in, for example, each cell of a memory, comparing an image of appearance of a pattern of one cell to that of a different another cell designed to have the same shape in the same chip and supposed to have the same appearance. The image of appearance of the pattern of the different another cell serves as a reference image.
In recent years, the number of defects to be reviewed which exist on the wafer is increased with increasing diameter of a semiconductor wafer. Since the review apparatus has low throughput as compared to a detection device, it is necessary to speed up the ADR operation which involves taking the defect image and the reference image, and identifying the defect part. Various speed-up methods have been proposed.
Generally, in the ADR, it takes much time to move a stage from an initial point to a destination, and to take the defect and reference images. In order to speed up the ADR, in addition of speeding up these processes, some of these image-taking steps may be omitted, which becomes effective. Normally, the step of taking the reference image may be omitted. For example, a method omitting one step has been proposed which involves previously preparing for the reference image. Alternatively, another omitting method has been proposed which involves synthesizing a reference image from a defect image, and inspecting the defect by comparing these images. The former in the cell-to-cell comparison involves previously storing a periodic pattern as an image for comparison, and comparing this image to the defect image, thereby extracting a defect area, as disclosed in JP-A No. 67243/2000. The latter involves comparing local areas or the like having the similar appearance on the defect image, and calculating the reliability of defect detection in a difference area based on a probability distribution of normal areas, thereby determining the difference area with high reliability as the defect, as disclosed in JP-A No. 98114/2003.
JP-A No. 325595/2001 discloses a method for extracting a defect part which involves binarizing a difference image between the defect image and the reference image, thereby extracting the defect part.